Today, a digital audio equipment that outputs music information recorded in a recording medium such as a CD or a DVD using a reproduction device such as a CD player or a DVD player is in widespread use. Since music information is recorded as digital information in a recording medium such as a CD in such a digital audio equipment, deterioration of the music quality accompanied by repeated reproduction/recording operation can be prevented, allowing a user to always enjoy high quality music. However, general digital information (music information) recorded in such a recording medium is limited to information within a frequency range that can be generally perceived by the human ear, and high-frequency music information higher than a predetermined frequency is deleted.
For example, it is said that the frequency that can be generally perceived by the human ear is from 20 Hz to 20 kHz. The sampling frequency used in a music CD is 44.1 kHz, and the frequency range that can be reproduced by this CD is 20 kHz or less. As described above, in a recording medium such as a CD, music information having a frequency higher than a frequency (20 kHz) that can be perceived by the human ear is deleted.
However, the human ear can perceive a high-frequency component higher than 20 kHz as a difference in tone. Therefore, many users say that sound output from the digital audio equipment in which the high-frequency component has been cut off has less richness or punch as compared to sound output from a conventional analog/audio equipment. Thus, today, there is proposed a method that interpolates a high-frequency audio signal in the music to be reproduced in the digital audio equipment so as to enhance a feeling of satisfaction of listeners (refer to, e.g., Patent Document 1).
A high-frequency interpolation device disclosed in Patent Document 1 performs up-sampling for a predetermined upper limit frequency in a high-frequency limited audio signal, i.e., applies zero-order interpolation to the center of the signal and then performs low-pass filtering processing so as to remove a high-frequency signal for down-sampling. Further, the high-frequency interpolation device performs envelope processing so as to allow the audio signal to have prescribed characteristics, thereby achieving high-frequency interpolation.
An application of such high-frequency interpolation enables removal of feeling of lack of high-frequency range at the time of sound reproduction.    Patent Document 1: JP-A-9-23127 (pages 2 and 3, FIG. 2)